The present invention relates to a method for coating a pump impeller so that the coating process is carried out without need for core structures.
In operation, the pump impeller as well as the pump housing of a pump are subject to various types of strain owing to the material to be pumped. One significant strain is the wear caused by the material. Especially when pumping sludge containing solid substances, the solids contained in the sludge cause substantial wear in the impeller and the pump housing. In order to resist wear, the impeller and the pump housing are generally protected by means of rubber-containing material; apart from protecting the impeller and the pump housing from wear, the rubber-containing material also protects them from corrosion caused by the liquid phase, i.e. water, of the sludge. The coating of the pump housing walls as such does not require special large-scale arrangements, because the pump housing walls are normally straight surfaces, and the wall junctions are formed to be curved surfaces in order to create advantageous flow conditions. The impeller, on the other hand, has a very complex surface and its coating requires special arrangements in order to spread an advantageous, even coating on all surfaces o the impeller, so that the shape of the impeller will be just as favorable hydrodynamically after coating as before.
In the prior art there is known a method for rubberizing a pump impeller, in which method complicated core structures are utilized in order to create an advantageous, even rubber coating. According to this previously known method, the ready-welded impeller frame, which is made of steel and comprises the impeller hub, the two side plates and the blade supports, is in the first stage placed within the coating mold suspended by special guides located in the mold. Thereafter the mold is filled with a sufficient amount of rubber and pressed, so that any excess rubber is squeezed out during the pressing. Consequently, the mold must have a solid structure in order to withstand the fairly high working pressure required in the rubberizing and the fairly high temperature caused by the molten rubber. In addition to the above factors, the structure of the mold becomes even more complicated owing to the steam channels which are necessary because the rubberizing process requires an essentially even temperature.
Although the above described, previously known rubberizing method aims at creating a coating as even as possible in order to fulfill the hydrodynamic requirements, the various stages of the process proceed, however, mainly according to the conditions set by the coating technique, and the hydrodynamic aspects receive less attention. Furthermore, because the mold must support the impeller, the mold is generally manufactured by machining from a solid block of material, and its manufacturing expenses are therefore high. Moreover, the currently used mold technique is not suited for serial production, but each impeller must be rubberized separately all through the process.
The present invention may be used to eliminate some of the drawbacks of the prior art and to achieve a better and simpler method for coating a pump impeller, so that the mold employed in the coating process is suited for serial production, and costly and complicated core structures are unnecessary.